SI-6726 Hash consing for Pattern matching Sym-s

For the test case: https://issues.scala-lang.org/browse/SI-6726?focusedCommentId=61207&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-61207

time scalac -Ystatistics -Ystop-after:patmat sandbox/gen.scala

Before:

    real    1m47.737s
    user    2m14.314s
    sys 0m2.783s

After:

    real    1m23.574s
    user    1m51.795s
    sys 0m2.634s
SI-6726 Further optimization of pattern analysis

Manually fuse `.filterNot(..).map` in dropUnit.

Before:

    real    1m23.574s
    user    1m51.795s
    sys 0m2.634s

After:

    real    1m4.883s
    user    1m30.754s
    sys 0m1.776s